This invention relates to highly sensitive accelerometers, and more particularly to a fiber optic based accelerometer.
It is known to monitor the physical characteristics of structures and bodies using sensors. One such application is the monitoring of oil wells to extract such information as temperature, pressure, fluid flow, seismic, and other physical characteristics. The monitoring of oil wells presents certain challenges for conventional sensors because they must be placed in harsh environments (e.g., high pressures and temperatures). Historically, such monitoring has been dominated by the use of electronic sensors and optical sensors to a lesser degree.
Such conventional electrical sensors are limited for several reasons. The on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration, and high levels of external hydrostatic pressure. Such electrical sensors also must be extremely reliable, since early failure entails very time consuming and expensive well intervention. Electronics, with its inherent complexity, are prone to many different modes of failure. Such failures have traditionally caused less than acceptable levels of reliability when these electrical sensors are used to monitor oil wells.
There are numerous other problems associated with the transmission of electrical signals within well bores. In general, it is difficult to provide an insulated electrical conductor for transmitting electrical signals within well bores. Such electrical conductors are extremely difficult to seal against exposure to well bore fluids, which are at high temperatures, high pressures, and present a very corrosive environment. Such electrical conductors, once damaged by the fluids that penetrate the insulating materials around the electrical conductors, will typically short electrical signals. Additionally, electrical transmissions are subject to electrical noises present in some production operations.
It is typical to use an accelerometer to measure downhole seismic disturbances to determine the acoustic wave characteristics of underground layers in the proximity of the well bore. An accelerometer is generally a mass-spring transducer housed in a sensor case. The sensor case is coupled to a moving body, the earth, whose motion is inferred from the relative motion between the mass and the sensor case. Such accelerometers relate the relative displacement of the mass with the acceleration of the case, and therefore the earth in the proximity of the well bore. An array of accelerometers is typically placed along the length of a well bore to determine a time dependent seismic profile.
One prior art accelerometer is a piezoelectric based electronic accelerometer. The piezoelectric based electronic accelerometer typically suffers from the above-referenced problems common to electrically based sensors. In particular, most high performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of such sensors is not only cumbersome but tends to occur at a significant increase in weight and volume of an accelerometer array, as well as a decrease in reliability. Also, piezoelectric accelerometers operate poorly at the lowest frequencies in the seismic band.
It is also known to use optical interferometer accelerometers to measure the acceleration of certain structures, and that they can be designed with fairly high responsivities and reasonably low threshold detection limits. Some prior art types of fiber optic accelerometers include interferometric fiber optic accelerometers based on linear and nonlinear transduction mechanisms, circular flexible disks, rubber mandrels, and liquid-filled-mandrels. Some of these fiber optic accelerometers have displayed very high acceleration sensitivity (up to 104 radians/g), but tend to utilize a sensor design that is impractical for many applications.
For instance, sensors with very high acceleration sensitivity typically often have a seismic mass greater than 500 grams. This seriously limits the frequency range in which the device may be operated as an accelerometer. The devices are so bulky that their weight and size renders them useless in many applications. Other fiber optic accelerometers suffer either from high cross-axis sensitivity or low resonant frequency, or require an ac dither signal, and tend to be bulky ( greater than 10 kg), expensive, and require extensive wiring and electronics. Even optical interferometers designed of special materials or construction are subject to inaccuracies because of the harsh borehole environment and the very tight tolerances present in such precision equipment.
For many applications, the fiber optic sensor is expected to have a flat frequency response up to several kHz (i.e., the device must have high resonant frequency) and high sensitivity. For many applications, the fiber optic sensor must be immune to extraneous measurands (e.g., dynamic pressure) and must have a small foot print and packaged volume that is easily configured in an array (i.e., easy multiplexing).
Objects of the present invention include provision of a fiber optic accelerometer for use within a harsh environment.
The invention may be used in harsh environments (high temperature, and/or pressure, and/or shock, and/or vibration), such as in oil and/or gas wells, engines, combustion chambers, etc. In one embodiment, the invention may be an all glass fiber optic sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications independent of the type of environment.
It is an object of the present invention to provide a highly sensitive linear accelerometer for sensing acceleration in a predetermined direction. The accelerometer is comprised of a rigid housing with a mass suspended therein by at least two elastic support members. The at least two elastic members are axially aligned in the predetermined direction, are attached to opposite ends of the housing, and are further attached to the mass. At least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to acceleration along the predetermined direction. Certain embodiments include a pair of fixed mandrels rigidly attached to opposite ends of the housing, and the mass comprises at least one floating mandrel wherein the elastic support members are each wrapped around one of the fixed mandrels and the floating mandrel.
It is another object of the present invention to provide a linear accelerometer where the mass comprises a pair of floating mandrels and wherein each elastic support member is wrapped about one of the fixed mandrels and one the floating mandrels. In another embodiment the mandrels and the mass of the accelerometer comprise a toroidal shape.
It is yet another object of the present invention to provide a linear accelerometer where at least one of the elastic support members comprises an optical fiber coil. The movement of the mass induces in the optical fiber coil a variation in length corresponding to the movement, allowing for interferometric measurement to determine the variation in length of the fiber.
It is still another object of the present invention to provide a linear accelerometer having an axial alignment assembly attached to the mass. The axial alignment assembly limits movement of the mass in a direction perpendicular to the predetermined direction. The axial alignment assembly comprises a flexure member attached to the mass and the housing. The flexure member allows axial movement of the mass in the predetermined direction and limits non-axial movement of the mass. In one embodiment, a pair of alignment assemblies are employed where the flexure member is a diaphragm positioned on an alignment rod and the diaphragm is captured within a bore in the housing about their outer periphery. Another embodiment provides for a bore positioned in the fixed mandrels for capturing the diaphragms. In another embodiment, the flexure member comprises a thin flexible plate and at least one pair of the flexure members are attached to the mass and to the housing.
It is still further an object of the present invention to provide a linear accelerometer where the transducer comprises a strain sensing element including a fiber optic strain sensor, a piezo electric device, a PVDF material, or a resistive strain gauge.
It is another object of the presenting invention to provide a highly sensitive linear accelerometer for sensing acceleration in a predetermined direction. The highly sensitive linear accelerometer has a rigid housing, a mass, a pair of fixed mandrels, two pairs of elastic support members, and a pair of axial alignment assemblies. The mass has an elongated body and rounded ends. The pair of fixed mandrels is rigidly attached to the housing and defines a predetermined distance therebetween. The two pairs of elastic support members are axially aligned in the predetermined direction and are wrapped around the fixed mandrels and the rounded ends in a continuous fashion to suspend the mass within the housing. At least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to acceleration along the predetermined direction. The pair of axial alignment assemblies is attached to the mass and limits movement of the mass in a direction perpendicular to the predetermined direction.
It is yet another object to provide a linear accelerometer where the fixed mandrels and the mass are comprised of a toroidal shape.
It is still another object of the present invention to provide an apparatus for vertical seismic profiling of an earth borehole having an x-direction, a y-direction, and a z-direction orthogonal to each other. The apparatus includes an optical fiber transmission cable and includes a plurality of linear accelerometers coupled to the borehole and in optical communication with the optical fiber transmission cable. The plurality of linear accelerometers are positioned in each of the three orthogonal directions. Each of the linear accelerometers is a highly sensitive linear accelerometer for sensing acceleration in a predetermined one of the directions. Each accelerometer includes a rigid housing, a mass, and at least two elastic support members. The at least two elastic support members are comprised of optical fiber axially aligned in the predetermined direction and attached to opposite ends of the housing and further attached to the mass. The elastic support members suspend the mass within the housing. At least a portion of one of the elastic support members comprises a transducer capable of measuring a displacement of the mass within the housing in response to an acceleration along the predetermined direction. The transducer is capable of providing a respective sensing light signal indicative of static and dynamic forces at a respective accelerometer location. The apparatus also includes an optical signal processor connected to the optical transmission cable for providing seismic profile information based on the respective sensing light signal.